1. Field of the Invention
The present invention relates to a piezoelectric ceramic having only a small dependence on temperature and exhibiting excellent piezoelectric characteristics, a method of manufacturing the same, and a piezoelectric/electrostrictive device having only a small dependence on temperature and exhibiting excellent piezoelectric characteristics.
2. Description of the Related Art
As a device capable of controlling a micro-displacement on the order of submicrons, a piezoelectric/electrostrictive device has heretofore been known. Especially, the piezoelectric/electrostrictive device includes a piezoelectric/electrostrictive portion constituted of a piezoelectric/electrostrictive porcelain composition (hereinafter simply referred to as the “piezoelectric ceramic”) and an electrode portion to which a voltage is to be applied, the portions being laminated on a substrate made of a ceramic. The device is suitable for control of micro-displacement, and additionally has excellent characteristics such as a high electromechanical coupling factor, a high-speed response, a high durability and a saved power consumption. Such a piezoelectric/electrostrictive device is used for various applications such as a piezoelectric pressure sensor, a probe movement mechanism of a scanning tunnel microscope, a rectilinear guide mechanism of an ultra-precise processing device, a servo valve for hydraulic control, a head of a VTR device, pixels constituting a flat panel type image display device and a head of an ink jet printer.
Moreover, the piezoelectric ceramic constituting the piezoelectric/electrostrictive portion has been variously investigated. For example, in recent years, the influence on the global environment by the elution of lead (Pb) due to acid rain has been regarded as a problem. Therefore, in consideration of the influence on the environment, an (LiNaK)(NbTa)O3-based piezoelectric ceramic has been developed which is capable of providing piezoelectric or a piezoelectric device exhibiting excellent piezoelectric/electrostrictive characteristics, even without containing any lead (Pb) (see, e.g., Patent Document 1).
Since the piezoelectric ceramic is a ferroelectric, a polarization treatment needs to be performed in order to utilize a property (a piezoelectric characteristic) of the ceramic incorporated in an electronic device and the like. This polarization treatment refers to a treatment of applying a high voltage to align a direction of spontaneous polarization in a specific direction. The treatment is performed by applying the voltage to the piezoelectric ceramic under appropriate temperature conditions and the like.
As a related conventional technology, a method is disclosed in which the polarization treatment is performed twice in different temperature ranges (see, e.g., Patent Document 2). Another method is disclosed in which heating is performed after the polarization treatment in order to increase a mechanical quality coefficient Qm of the resultant piezoelectric ceramic (see, e.g., Patent Document 3). Furthermore, there are disclosed an oriented crystal ceramic and a method of manufacturing the ceramic constituted of a polycrystalline including a predetermined isotropic perovskite type compound as a main phase and having specific crystal oriented (see, e.g., Patent Document 4).
However, even when the piezoelectric ceramic is subjected to the polarization treatment and the like by the methods disclosed in Patent Documents 2 to 4, a large dependence on temperature raises the problem that the piezoelectric characteristic easily changes with the temperature. The piezoelectric/electrostrictive device in which the piezoelectric ceramic having the large dependence on the temperature is used has the problem that the response in the case where the temperature conditions change easily becomes unstable, and it is difficult to adopt the device in an application requiring a finer displacement.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-221276
[Patent Document 2] Japanese Patent Application Laid-Open No. 228865
[Patent Document 3] Japanese Patent Application Laid-Open No. 2004-2051
[Patent Document 4] Japanese Patent Application Laid-Open No. 2004-300019